Human immunodeficiency virus is a human retrovirus and is the etiological agent of acquired immunodeficiency syndrome (AIDS). Since AIDS was first reported in the US in 1981, more than 194,000 people have died of AIDS and over 330,000 cases of HIV infection have been reported in the US alone. Worldwide, it is estimated that more than 17 million people have been infected with HIV.
More than 100 AIDS-related medicines are in human clinical trials or awaiting FDA approval but there is currently no cure for the disease.
There is, therefore, a clear need for immunogenic preparations useful as vaccine candidates, as antigens in diagnostic assays and kits and for the generation of immunological reagents for diagnosis of HIV and other retroviral disease and infection.
Particular prior art immunogenic preparations include non-infectious, non-replicating HIV-like particles. Thus PCT applications WO 93/20220 published Oct. 14, 1993 and WO 91/05860 published May 2, 1990 (Whitehead Institute for Biomedical Research), teach constructs comprising HIV genomes having an alteration in a nucleotide sequence which is critical for genomic RNA packaging, and the production of non-infectious immunogenic HIV particles produced by expression of these constructs in mammalian cells.
PCT application WO 91/07425 published May 30, 1991 (Oncogen Limited Partnership) teaches non-replicating retroviral particles produced by co-expression of mature retroviral core and envelope structural proteins, such that the expressed retroviral proteins assemble into budding retroviral particles. A particular non-replicating HIV-1 like particle was made by coinfecting mammalian host cells with a recombinant vaccinia virus carrying the HIV-1 gag and protease genes and a recombinant vaccinia virus carrying the HIV-1 env gene.
In published PCT application WO 91/05864 in the name of the assignee hereof (which is incorporated herein by reference thereto), there are described particular non-infectious, non-replicating retrovirus-like particles containing at least gag, pol and env proteins in their natural conformation and encoded by a modified retroviral genome deficient in long terminal repeats and containing gag, pol and env genes in their natural genomic arrangement.
Virions of HIV comprise two copies of the single-stranded RNA genome enclosed within a capsid. After penetration into a susceptible host cell, the HIV genome is copied by the viral reverse transcriptase into single-stranded DNA that is thought to be translocated into the nucleus, wherein a cellular DNA polymerase synthesizes the second DNA strand. The double-stranded copy is then integrated, at random, into one of the host chromosomes, resulting in a duplication of a region of the viral genome at the extremities of the genome. The long-terminal repeat (LTR) of the integrated provirus is recognized by a cellular RNA polymerase and the transcribed RNA is translated to give rise to viral proteins. The RNA transcripts can also be packaged into new virions that leave the cell by a process of budding.
The HIV genome encodes at least nine different proteins. The three major genes, gag, pol and env are common to all retroviruses and encode virion proteins.
The differential expression of these genes is achieved through a complex pattern of processing of the primary precursor transcript. Only the GAG and POL proteins are produced from the unspliced mRNA corresponding to the genomic RNA of the virion. The ENV protein is translated from a mRNA species that has undergone a single splicing event to delete the gag and pol coding sequences, and other proteins are produced from mRNA species that are spliced several times. The general structure of HIV is reviewed by Kieny et al (ref. 8).
Thus, it may be advantageous under particular circumstances to produce retrovirus-like particles (and in particular HIV-like particles) by mutating other portions of the HIV genome contributing to infectivity and replication of the virus. Such modifications may be modifications of the gag and pol gene products.
There is currently no vaccine nor effective treatment for AIDS. Heat-inactivated anti-HIV antiserum obtained from HIV-infected people and inactivated HIV are currently commercially available as components of many diagnostic methods. For safety, ease of handling, shipping, storage and use, it may be preferable to replace such antigen and heat-inactivated antisera by non-infectious HIV-like particles and antisera generated by immunization with non-infectious HIV-like particles as described above and particularly in WO 91/05864. Furthermore, antisera generated by immunization with these non-infectious HIV particles do not require heat inactivation to remove infectious HIV. The HIV-like particles described in WO 91/05864 are entirely deficient in replication and infection. However, because of the seriousness of HIV infection, it may be desirable under certain circumstances to provide retrovirus-like particles deficient in a plurality of elements required for infectivity and/or replication of HIV but dispensible for virus-like particle formation. Furthermore, since prior art HIV-like particles contain many of the HIV proteins in substantially their natural conformations, a host immunized therewith may mount an immune response immunologically indistinguishable from infection by HIV and it may be desirable to be able to distinguish between inactivated HIV and non-infectious, non-replicating HIV particles and antisera generated by virulent HIV and non-infectious, non-replicating HIV-like particles. Thus, in the development of AIDS vaccine candidates, immunogenic preparations and diagnostic methods and kits, it would be useful to provide an HIV-like particle deficient in a plurality of elements required for infectivity and/or replication and optionally immunologically or otherwise distinguishable from virulent HIV.